In the gasification of solid carbonaceous source materials such as coal, char, coke, and the like, a metal oxide such as zinc oxide has been employed to provide at least part of the oxygen required. Such techniques are disclosed in U.S. Pat. Nos. 2,592,377 and 2,602,809.
In these processes, a finely divided solid particle form carbonaceous material is admixed with zinc oxide as the oxygen carrier or oxygen donor according to the basic reaction: EQU C+ZnO.fwdarw.Zn+CO.
Formation of Zn vapor occurs in the gasification of the carbonaceous material such as char when using ZnO as the oxygen carrier or oxygen donor. It is necessary to cool the reaction products, which are gaseous, subsequent to the gasification. This leads to separation of Zn as a molten metal, and the gaseous CO, carbon monoxide, as the desired product.
Unfortunately, during cooling, formation of some zinc oxide also has been a partial result. Apparently, some reversion to the oxide of zinc occurs with consequent loss of a portion of the desired carbon monoxide. It is believed that zinc oxide forms due to the reversible reaction between zinc and traces of carbon dioxide, also present in the reaction product mixture in small concentrations: EQU Zn(g)+CO.sub.2 (g).fwdarw.ZnO(s)+CO(g)
The so-formed zinc oxide is an undesirable product since the zinc oxide tends to form a coating around droplets of molten zinc, depositing as a "blue powder" in zinc collection chambers, leading to line plugging, and preventing subsequent reoxidation of the metallic zinc inside the droplets for reuse. Excessive amounts of solid zinc oxide mixed with molten zinc create serious problems of material transport and handling of the molten zinc. U.S. Pat. No. 2,342,368 teaches a process for reduction of zinc oxide by carbonaceous materials, teaching one effort to reduce or prevent formation of blue powder, but recycles carbon monoxide to the feed and to the zinc oxide/char reactor. Blue powder has been a perennial problem in zinc handling for a long time. For example, the Encyclopedia of Chemical Technology (1970) in Vol. 22, page 579, in describing zinc metallurgy suggests chilling zinc vapor to avoid formation of blue powder, though there is no mention of a char gasification reaction.